Various methods and apparatus are available for amplifying signals. In amplifier applications that involve the amplification and transmission of modulated signals, a premium is placed on amplifier efficiency. In addition, because many modulated signals contain information in the amplitude envelope and the phase of the signal, a premium is placed on the ability to create a high fidelity reproduction of the signal. In particular, an amplifier preferably exhibits very low amplitude distortion and very low phase distortion.
Communications devices, which often transmit signals having information in both amplitude and phase, are an example of an application where these qualities are in demand. Low distortion allows the communications devices to communicate more reliably, and high efficiency allows the devices to operate longer on a single battery.
One method of achieving increased efficiency is to use envelope elimination and restoration ("EER")-type amplifiers. EER is a technique through which highly efficient but nonlinear radio frequency ("rf") power can be combined with other, highly efficient amplifiers to produce a highly efficient linear amplifier system. The signal to be amplified is split into two paths: an amplitude path, and a phase path. The detected envelope is amplified efficiently in the amplitude path by a class S or other highly efficient power amplifier which only has to operate on the bandwidth of the rf envelope. The phase modulated carrier in the phase path is then amplitude modulated by the amplified envelope signal, creating an amplified replica of the input signal.
It is desirable to provide an EER amplifier with high efficiency.